as far as I know the novel was completed long before, but published after her death (the first publisher wouldn't publish it for years).

She seems to have not said anything about this novel or the characters - such a pity.

In fact in the last chapter she does mention this sudden ending;

... my readers, who will see in the tell–tale compression of the pages before them, that we are all hastening together to perfect felicity.

So it isn't really an incomplete novel like some others.
It would seem JA wanted to say just this much and no more, (parody and all about the defense of novel etc.) and wasn't trying to develop the romantic angle - she just wanted to write something opposite to gothic novels, and anything more would be, - well,... a gothic novel ;-)